Optical disks are used in many fields such as audio equipment, VCR, computers as an information recording medium that enables to record many information signals in the high density. In order to read out information signal, which has been recorded in micrometer unit, in the information recording medium such as an optical disk, it is necessary to accurately carry out tracking of a light beam with respect to a track on an optical disk. There are well-known various methods for detecting of a tracking error signals for carrying out the tracking (hereinafter referred to as TES).
One of the tracking methods is a difference push-pull method (hereinafter referred to as DPP method). According to the DPP method, an offset in TES, caused by a shifting of the object glass and by a tilting of an optical disk, is corrected by finding a difference between a push-pull signal (hereinafter referred to as a PP signal) of a main beam and PP signals of sub-beams. The main beam and the sub-beams are generated by a three-beam diffraction grating.
In the DPP method, in order to cancel an offset, a phase difference of sub-beam PP signals is set to 180° with respect to the main beam PP signal. However, in order to provide the phase difference of 180°, it is necessary to carry out respective position adjustments of the main beam and the sub-beams that are converged on the track on the optical disk. During the adjusting, it is necessary to carry out an accurate rotation adjustment or the like of the three-beam diffraction grating.
In view of the circumstances, the DPP method is improved, and a phase shift DPP method is proposed. In the phase shift DPP, the position adjustments of the main beam and the sub-beams converged on a track of an optical disk are omitted. This allows the simplification of adjustments during assembling of an optical pickup. The phase shift DPP is disclosed in Japanese unexamined patent publication No. 2001-250250 (Tokukai 2001-250250; published on Sep. 14, 2001).
As shown in FIG. 16, according to the phase shift DPP method disclosed in the Tokukai 2001-250250, laser light emitted from a light source 101 is converted to parallel light via a collimator lens 102, and is divided into a main beam 130, a sub-beam (positive first order light beam) 131, and a sub-beam (negative first order light beam) 132, via a diffraction grating 103. These three beams are converged onto a track on an optical disk 106 by an objective lens 105 after passing through a beam splitter 104. The light reflected from the optical disk 106 is further reflected by a beam splitter 104 via the objective lens 105, and is directed to a photo detector 108 (108A, 108B, 108C), via a collective lens 107. As such, far field patterns of the reflected light of the main beam 130 and the sub-beams 131 and 132 are directed to a two-division photo-detectors 108A, 108B, and 108C.
Here, it is assumed that an original point is at a center of the beam, x-axis extends in a radial direction of the optical disk, and y-axis extends in a track direction. In the diffraction grating 103, a periodical structure of the grating grooves of the first quadrant has a phase difference of 180° from that of the grating grooves of the second through fourth quadrant. This causes the sub-beams 131 and 132, diffracted by the grating groove, to have a phase difference of 180° in the first quadrant. As such, as shown in FIG. 17(a), difference signals outputted from the two-division photo-detectors 108B and 108C, i.e., the PP signals 131 and 132 of the sub-beams are substantially zero (0) in their amplitudes, compared to the PP signal PP130 of the main beam, i.e., the difference signal from the two-division photo-detector 108A without phase difference.
The PP signals derived from the sub-beams 131, 132 are not detected regardless of the track position. Therefore, substantially the same differential PP signals are obtained when the sub-beams 131 and 132 are directed onto the same track where the main beam 130 is directed, and when the sub-beams 131 and 132 are directed onto a different track, respectively.
On the other hand, as shown in FIG. 17(b), as to an offset in TES caused by tilting of the objective lens 105 or the optical disk 106, the PP signals 130 and 131 (PP signal 132) have common mode offsets of Δp and Δp′ in accordance with the light amount. As such, a differential PP signal 134 that has been subject to the cancellation of the offsets can be calculated and detected using the following formula.PP134=PP130−k(PP131+PP132)=PP130−k·PP133  (1)
Note that a coefficient k in the formula (1) is for correcting the difference of the light intensity between (i) the zero order light main beam and (ii) the positive and negative first order light sub-beams 131 and 132. For example, when the ratio of the light intensity is expressed by zero order light: positive first order light (+): negative first order light (−)=a:b:b, the coefficient k satisfies k=a/(2b). Moreover, as shown in the formula (1), the PP signal 133 is the sum of the PP signals of the sub-beam 131 and the sub-beam 132.
In this way, the sum (the PP signal 133) of the sub-beam PP signals 131 and 132 has amplitude of 0 regardless of groove depth. Since the amplitude is thus 0 regardless of the position of the beams on the track, it is not necessary to carry out a position adjustment, such as the rotation adjustment of the diffraction grating, for the three beams. As such, it is possible to greatly simplify the assembling adjustment of the pickup.
However, in case of adopting an optical pickup utilizing a phase shifted diffraction grating, the offset sensitivity (lens shift signal) for the PP signals of the sub-beams that indicates a shifting of the objective lens (hereinafter referred to as lens shift) may not change linearly with respect to the lens shift of the objective lens.
More specifically, when center values of the of the PP signals of the respective amplitude (values corresponding to Δp and Δp′ shown in FIG. 17 (b)) are used to evaluate an offset value for the lens shift, the lens shift signal may change as shown in FIG. 18. In other words, the lens shift signals for the main beam PP signals (a solid line in FIG. 18) change linearly, whereas lens shift signals for the sub-beam PP signals (a dotted line in FIG. 18) may change nonlinearly.
The nonlinear change in the lens shift signals of the sub-beam PP signals is attributed to structures around a border between regions, in the diffraction grating, that have different phases from one another. In other words, the lens shift signals of the sub-beam PP signals are easy to be affected around the border by an interference of the light beams that have passed through the regions having their respective phases and/or by a deviation of the grating groove around the border from a realistic design state etc. This causes nonuniformity of the amplitude of the lens shift signals of the PP signals of the sub-beams in accordance with position relation between regions around the border and a division line of the photo-detector (see reference numeral 108 in FIG. 16), thereby causing the lens shift signal to change nonlinearly with respect to the lens shift.
If a rapid change occurs in offset sensitivity of PP signals as described above, the correction of an offset in TES becomes inaccurate, then it becomes difficult to carry out better tracking servo with respect to an optical pickup. Thus, an optical pickup utilizing a phase shift diffraction grating leaves room for an improvement in reliability in tracking servo.